Vesta, the planet that might have been

This is Vesta, the second largest asteroid in our solar system's main asteroid belt. Specifically, this is a view of Vesta's south pole, taken by NASA's Dawn spacecraft last September.

As it turns out, Vesta is a great illustration of the power of chance in the universe. Data collected by Dawn is showing that, once upon a time, this asteroid was on its way to planethood. But, for several reasons, it simply never grew large enough. From Science News:

... according to Dawn observations, Vesta did indeed agglomerate enough rocky debris as it grew to heat itself by the decay of the rock's radioactive elements. That heat led to the separation of the primordial body into a rocky crust, an underlying rocky mantle, and a central metallic core, hallmarks of planet Earth and the other rocky planets. Dawn was the first to detect Vesta's now-solid core.

Vesta isn't unique in this, but it does provide an interesting moment to stop and think a little bit about randomness and the process of planetary birth. This news about Vesta is a nice reminder that there's really no reason why our solar system has to have eight planets. It could have had fewer. It could have had more. And some bodies—like Ceres and Pluto—are really only a trick of taxonomy away from being planets.

And some bodies—like Ceres and Pluto—are really only a trick of taxonomy away from being planets.

This misses the whole difference between these objects and the eight major bodies.

Ceres, Vesta, and Pluto could have ended up bigger or smaller, but still come from regions where material remained fragmented into hundreds or thousands of pieces. None of them were on their way to becoming anything like even Mercury or Mars, where almost everything available aggregated into a single body.

The result is a substantial difference; rocky Mercury is 25 times the mass of even Pluto, though that formed far enough out to collect ice, and some 360 times Ceres. And it’s not entirely random, since once Jupiter formed, it would have prevented the aggregation of another planet so close.

The solar system isn’t just a collection of random objects. Whatever you think of taxonomy, if you want to understand its structure, there’s a major split here to keep in mind. There could have been more planets, but Vesta was never going to be one.